[docs] update roadmap document
[docs] update roadmap document

Updating the existing long-term roadmap to reflect the current status of the items on the roadmap. Note this roadmap could probably use a bit of a refresh, will add a discussion topic to the next SIG meeting agenda.

open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector